Chip-on-Substrate Packaging on Carrier

ABSTRACT

A method includes mounting a wafer-level package substrate over a carrier, and pre-cutting the wafer-level package substrate to form trenches extending from a top surface of the wafer-level package substrate into the wafer-level package substrate. A plurality of dies is bonded over the wafer-level package substrate. The plurality of dies is molded in a molding material to form a wafer-level package, with the wafer-level package including the wafer-level package substrate, the plurality of dies, and the molding material. The carrier is detached from the wafer-level package. The wafer-level package is sawed into a plurality of packages, with each of the plurality of packages including a portion of the wafer-level package substrate and one of the plurality of dies.

BACKGROUND

In the packaging of integrated circuits, device dies are packaged ontopackage substrates, which include metal connections that are used toroute electrical signals between opposite sides of the packagesubstrates. The device dies may be bonded onto one side of a packagesubstrate using flip chip bonding, and a reflow is performed to melt thesolder balls that interconnect the dies and the package substrate.

The package substrates may use organic materials such as materials thatcan be easily laminated. These materials, however, are prone to thewarpage caused by the elevated temperatures used in the reflow of thesolder. Furthermore, during the bonding process, since the dies and thepackage substrates have significantly different Coefficients of ThermalExpansion (CTEs), the warpage in the device dies and the packagesubstrates is worsened. For example, the silicon in the dies may have aCTE of 3.2 ppm/° C., while the package substrates may have a CTE betweenabout 17 ppm/° C. and 20 ppm/° C., or even higher. The warpage in thepackage substrates may cause irregular joints and/or bump cracks. As aresult, the yield of the packaging process is adversely affected.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the embodiments, and the advantagesthereof, reference is now made to the following descriptions taken inconjunction with the accompanying drawings, in which:

FIGS. 1 through 8 are cross-sectional views of intermediate stages inthe formation of a package in accordance with some exemplaryembodiments, wherein a wafer-level package substrate is adhered on acarrier and pre-cut before device dies are bonded thereon.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The making and using of the embodiments of the disclosure are discussedin detail below. It should be appreciated, however, that the embodimentsprovide many applicable concepts that can be embodied in a wide varietyof specific contexts. The specific embodiments discussed areillustrative, and do not limit the scope of the disclosure.

A Chip-on-Substrate (CoS) package (which may also be aChip-on-Chip-on-Substrate (CoCoS) package) and the method of forming thesame are provided. The intermediate stages of forming the package inaccordance with some embodiments are illustrated. The variations of theembodiments are discussed. Throughout the various views and illustrativeembodiments, like reference numbers are used to designate like elements.

FIGS. 1 through 8 illustrate the cross-sectional views and a top view ofintermediate stages in the formation of a package in accordance withsome exemplary embodiments. Referring to FIG. 1, wafer-level packagesubstrate 20 is adhered on carrier 22 through adhesive 23. In someembodiments, carrier 22 is a glass carrier, a ceramic carrier, or thelike, which may be formed of a homogenous material. Alternatively,carrier 22 is a blank silicon carrier that has no integrated circuitsformed thereon. Adhesive 23 may be a Ultra-Violet (UV) adhesive, whichloses adhesion when exposed to UV light.

In some exemplary embodiments, wafer-level package substrate 20 is abuild-up substrate that is built up from a core. In alternativeembodiments, wafer-level package substrate 20 is a laminate substratethat includes conductive traces embedded in laminated dielectric films.In the subsequent discussion of the embodiments of the presentdisclosure, a build-up substrate is illustrated as an example, while theteaching revealed in accordance with the exemplary embodiments arereadily applicable for laminate substrates.

In the exemplary embodiments that wafer-level package substrate 20 is abuild-up substrate, wafer-level package substrate 20 includes core 24,and metal layers formed on the opposite sides of core 24. Throughout thedescription, the term “metal layer” refers to the collection of allmetal features, including, and not limited to, metal traces and metalpads, that are at the same level. The corresponding parts/layers of themetal features in the same metal layer are formed of the same materialsthat have the same compositions. For example, all metal features in ametal layer may include a copper layer. Alternatively, a metal layerincludes a copper layer and a nickel layer over the copper layer. Insome exemplary embodiments as shown in FIG. 1, wafer-level packagesubstrate 20 includes metal layers L2 overlying core 24, and metal layerL1 over metal layer L2. Wafer-level package substrate 20 furtherincludes metal layer L3 underlying core 24, and metal layer L4underlying metal layer L3.

Package substrate 20 may include one or more metal layers on each sideof core 24. For example, in the exemplary embodiments shown in FIG. 1,there are two metal layers on each side of core 24. Metal layers L1, L2,L3, and L4 are electrically interconnected through metal vias 25, 26,and 28. Accordingly, metal traces 30, which are parts of metal layer L1,are electrically connected to metal pads 32, which are parts of metallayer L4. The metal features in metal layers L1, L2, L3, and L4 maycomprise copper, aluminum, nickel, gold, or combinations thereof. Core24 includes core dielectric layer 27, and metal vias 28 penetratingthrough core dielectric layer 27. In some embodiments, core dielectriclayer 27 comprises one or more material selected from epoxy, resin,glass fiber, molding compound, plastic (such as PolyVinylChloride (PVC),Acrylonitril, Butadiene & Styrene (ABS), Polypropylene (PP),Polyethylene (PE), PolyStyrene (PS), Polymethyl Methacrylate (PMMA),Polyethylene Terephthalate (PET), Polycarbonates (PC), Polyphenylenesulfide (PPS), combinations thereof, and multi-layers thereof. Metalvias 28 may be formed as conductive pipes in some exemplary embodiments.The internal volumes of metal vias 28 are filled with dielectric filling29, which may be a material selected from the same candidate materialsfor forming core dielectric layer 27. In alternative embodiments,conductive pipes 28 comprise air gaps therein. Metal vias 28electrically interconnect, and may be in physical contact with, themetal features in metal layers L2 and L3.

Dielectric layer 38 is formed over core 24, with the vias 26 thatinterconnect metal layers L1 and L2 disposed in dielectric layer 38.Dielectric layer 38 may be formed of PP in some exemplary embodiments,while other dielectric materials may also be used. Dielectric layer 40is formed under core 24, with the vias 25 that interconnect metal layersL3 and L4 disposed in dielectric layer 40. Dielectric layer 40 may alsobe formed of PP in some exemplary embodiments, while other dielectricmaterials may also be used.

Referring to FIG. 2A, a pre-cut is performed to cut through some partsof wafer-level package substrate 20, so that trenches 42 are formed. Insome embodiments, the pre-cut is a partial pre-cut, wherein a topportion of wafer-level package substrate 20 is cut, and a bottom portionof wafer-level package substrate 20 is not cut, as illustrated in FIG.2A. Accordingly, bottom surfaces 42A of trenches 42 are at anintermediate level between the top surface and the bottom surface ofwafer-level package substrate 20. In alternative embodiments, thepre-cut is a full pre-cut, wherein after the pre-cut, wafer-levelpackage substrate 20 is cut through. In some embodiments in which thefull pre-cut is performed, bottom surfaces 42A of trenches 42 are levelwith top surface 23A of adhesive 23. In alternative embodiments, bottomsurfaces 42A (also shown as dashed) of trenches 42 are at anintermediate level between top surface 23A and bottom surface 23B ofadhesive 23. In yet other embodiments, adhesive 23 is substantially cutthrough in the pre-cut step, and bottom surfaces 42A of trenches 42 aresubstantially level with bottom surfaces 23B of adhesive 23. The pre-cutmay be performed using laser or a blade. In some embodiments, bottomsurfaces 42A are substantially planar, and may be parallel to the majortop surface and bottom surface of package substrate 20.

Since the material of wafer-level package substrate 20 is different fromthe material of carrier 22, wafer-level package substrate 20 and carrier22 have different Coefficients of Thermal Expansion (CTEs). The CTE ofwafer-level package substrate 20 may be significantly greater than theCTE of carrier 22. For example, carrier 22 may have a CTE equal to about3 ppm/° C., while wafer-level package substrate 20 may have a CTEgreater than about 10 ppm/° C. With the difference between the CTEs ofcarrier 22 and wafer-level package substrate 20, stresses may begenerated in wafer-level package substrate 20. The pre-cut helps releasethe stress.

In some exemplary embodiments, trenches 42 extend below the bottomsurface 24A of core 24. Furthermore, depth D1 of trenches 42 may begreater than about 50 percent thickness T1 of package substrate 20.Depth D1 may also be in the range between about 50 percent and about 80percent thickness T1. It is appreciated, however, that the valuesrecited throughout the description are merely examples, and may bechanged to different values. Since core dielectric layer 27 in core 24may be more rigid (with a higher hardness) compared to other dielectriclayers in wafer-level package substrate 20, sawing through core 24improves the stress release effect of wafer-level package substrate 20.

FIG. 2B illustrates a top view of FIG. 2A. In some embodiments, as shownin FIG. 2A, wafer-level package substrate 20 and carrier 22 have a roundshape. The sizes of wafer-level package substrate 20 and carrier 22 maybe the same as the common sizes of semiconductor wafers, which sizes maybe 8 inches, 12 inches, or the like. Trenches 42 form a grid, with aplurality of trenches 42 extending parallel to the X direction, and aplurality of trenches 42 extending parallel to the Y direction. Trenches42 define package substrates 20′, which are portions of wafer-levelpackage substrate 20.

Referring to FIG. 3, dies 44 and 46 are bonded to wafer-level packagesubstrate 20. Package components 44 and 46 may be device dies, which mayinclude logic device dies, memory device dies, or the like. Hence,throughout the description, package components 44 and 46 are referred toas device dies 44 and 46, respectively, although they may also be othertypes of package components such as packages, for example. In someembodiments, device dies 44 and 46 include integrated circuit devices(not shown), such as transistors, capacitors, inductors, resistors, andthe like, therein. Device dies 44 may include semiconductor substrate45, and Through Vias (TV, sometimes referred to as through-silicon viasor through-semiconductor vias) 47 penetrating through semiconductorsubstrate 45. Accordingly, the integrated circuit devices in device dies46 may be electrically coupled to (as illustrated in FIG. 3) theconductive features in wafer-level package substrate 20 through TVs 47.

The bonding between dies 44 and wafer-level package substrate 20 and thebonding between dies 46 and dies 44 may be solder bonding or directmetal-to-metal (such as a copper-to-copper) bonding. Underfill 50 isdispensed into the gaps between dies 44 and wafer-level packagesubstrate 20, and underfill 52 is dispensed into the gaps between dies44 and 46. The resulting structure shown in FIG. 3 is referred to as aChip-on-Chip-on-Substrate (CoCoS) package. In alternative embodiments,there is no die over and bonded to dies 44, and the resulting package isa Chip-on-Substrate (CoS) package.

In some embodiments, chips 46 extend beyond the edges of dies 44, andhence dies 46 are referred to as having overhang. FIG. 3 illustratesthat a die 46 has an overhang on one side of the respective underlyingdie 44. In alternative embodiments, dies 46 may overhang on oppositesides of the respective underlying dies 44.

Referring to FIG. 4, molding material 54 (which may be a polymer) ismolded onto dies 44 and 46 and wafer-level package substrate 20. In someembodiments, polymer 54 comprises a molding compound, and hence isreferred to as molding compound 54 hereinafter, although it may also beformed of other types of polymers such as epoxy, molding underfill, orthe like. Molding compound 54 is further filled into trenches 42. Afterthe curing of molding compound 54, a grinding step may be performed onmolding compound 54 to level the top surface of molding compound 54. Inthe resulting structure, top surface 54A of molding compound 54 may behigher than or level with top surfaces 46A of dies 46.

In alternative embodiments, instead of applying underfills 50 and 52 andmolding compound 54 in separate steps, wherein the dispensing of each ofunderfills 50 and 52 and the molding compound 54 is accompanied by acuring process, a molding underfill is used to replace underfills 50 and52 and molding compound 54. Accordingly, the molding underfill fills thegaps between dies 44 and 46 and wafer-level package substrate 20, andalso fills the gaps between dies 44 and 46. After the dispensing of themolding underfill, a curing step is performed. Throughout thedescription, the structure including wafer-level package substrate 20,dies 44 and 46, and the molding/underfill material in FIG. 4 is referredto as wafer-level package 56.

Next, wafer-level package 56 is demounted from carrier 22. For example,in the embodiments wherein adhesive 23 (FIG. 4) is a UV glue, a UV lightpenetrates through carrier 22, which is transparent to the UV light, andis projected on adhesive 23. Adhesive 23 loses adhesion due to the UVlight. The residue of adhesive 23 left on wafer-level package substrate20 is then cleaned. FIG. 5 illustrates the resulting wafer-level package56.

FIG. 6 illustrates the formation of connectors 58. The respective stepis sometimes referred to as ball mount. In some exemplary embodiments,the formation of connectors 58 includes placing solder balls on theexposed portions of the conductive features (such as traces 32) onwafer-level package 56, and then reflowing the solder balls. Inalternative embodiments, the formation of connectors 58 includesperforming a plating step to form solder regions on the conductivefeatures on wafer-level package 56, and then reflowing the solderregions to form solder balls. Connectors 58 may also comprise metalpillars, or metal pillars and solder caps, which may also be formedthrough plating.

After the formation of connectors 58, as shown in FIG. 7, a die-saw isperformed to saw wafer-level package 56 apart, so that packages 60 areformed. Each of packages 60 includes a portion (referred to as packagesubstrate 20′ hereinafter) of wafer-level package substrate 20, and dies44 and 46 bonded thereon. In some embodiments, kerfs 62 of the die-saware narrower than trenches 42 (FIGS. 2A and 2B). As a result, portionsof molding compound 54 that fills trenches 42 are left un-removed by thedie saw. In alternative embodiments, kerfs 62 of the die-saw have thesame width as trenches 42 (FIG. 2). In yet alternative embodiments,kerfs 62 of the die-saw are wider than trenches 42 (FIG. 2), and henceno molding compound 54 is left on the sidewalls of package substrates20′.

FIG. 8 illustrates package 60 in accordance with the embodiments of thepresent disclosure. As shown in FIG. 8, molding compound 54 includesportion 54A encircling dies 44 and 46. Portion 54A of molding compound54 is further in contact with the sidewalls of dies 44 and 46. Moldingcompound 54 further includes portion 54B encircling package substrate20′. Portion 54B of molding compound 54 is further in contact with thesidewalls of the top portion of package substrate 20′. On the otherhand, the bottom surface of molding compound portion 54B is in contactwith a top surface of dielectric layer 40 in package substrate 20′.Accordingly, step 64 is formed, as shown in the cross-sectional view ofpackage 60. The step 64 includes bottom surface 54A1 of molding compoundportion 54A, bottom surface 54B1 of molding compound portion 54B, andsidewall 54C of molding compound portion 54B. Due to the step, moldingcompound 54 has an improved adhesion to package substrate 20′ than ifstep 64 is not formed. Furthermore, Portion 54B of molding compound 54encircles, and in contact with, core 24 of package substrate 20′ in someembodiments.

The embodiments of the present disclosure have some advantageousfeatures. By forming the CoCos package on a carrier, it is possible topre-cut the package substrate before bonding dies onto the packagesubstrate. Accordingly, the stress in the package substrate has alreadybeen released when the bonding is performed. This results in theimproved reliability in the resulting package and reduced difficulty inthe bonding. The formation of the step helps prevent molding compound 54from delaminating from package substrate 20′.

The carrier and the package substrate may have top-view shapes and sizesthe same as semiconductor wafers. Accordingly, the tools and the processsteps that are used for forming devices on semiconductor wafers arereadily usable for the formation of the CoCoS packages. Since thepackaging is performed at the wafer level, there is better processuniformity and quality control.

In accordance with some embodiments, a method includes mounting awafer-level package substrate over a carrier, and pre-cutting thewafer-level package substrate to form trenches extending from a topsurface of the wafer-level package substrate into the wafer-levelpackage substrate. A plurality of dies is bonded over the wafer-levelpackage substrate. The plurality of dies is molded in a molding materialto form a wafer-level package, with the wafer-level package includingthe wafer-level package substrate, the plurality of dies, and themolding material. The carrier is detached from the wafer-level package.The wafer-level package is sawed into a plurality of packages, with eachof the plurality of packages including a portion of the wafer-levelpackage substrate and one of the plurality of dies.

In accordance with other embodiments, a method includes performing apartial pre-cutting on a wafer-level package substrate to form trenchesin the wafer-level package substrate, wherein bottom surfaces of thetrenches are between a top surface and a bottom surface of thewafer-level package substrate. After the pre-cutting, a plurality ofdies is bonded over the wafer-level package substrate. The plurality ofdies is molded in a molding material to form a wafer-level package, withthe wafer-level package including the wafer-level package substrate, theplurality of dies, and the molding material. The wafer-level package issawed into a plurality of packages.

In accordance with yet other embodiments, a package includes a packagesubstrate having a top surface and a bottom surface, a die over andbonded to the package substrate, and a molding material molding the dietherein. The molding material includes a first portion encircling thefirst die, and a second portion underlying and connected to the firstportion. The second portion encircles a top portion of the packagesubstrate. A bottom surface of the second portion of the moldingmaterial is at an intermediate level between the top surface and thebottom surface of the package substrate.

Although the embodiments and their advantages have been described indetail, it should be understood that various changes, substitutions andalterations can be made herein without departing from the spirit andscope of the embodiments as defined by the appended claims. Moreover,the scope of the present application is not intended to be limited tothe particular embodiments of the process, machine, manufacture, andcomposition of matter, means, methods and steps described in thespecification. As one of ordinary skill in the art will readilyappreciate from the disclosure, processes, machines, manufacture,compositions of matter, means, methods, or steps, presently existing orlater to be developed, that perform substantially the same function orachieve substantially the same result as the corresponding embodimentsdescribed herein may be utilized according to the disclosure.Accordingly, the appended claims are intended to include within theirscope such processes, machines, manufacture, compositions of matter,means, methods, or steps. In addition, each claim constitutes a separateembodiment, and the combination of various claims and embodiments arewithin the scope of the disclosure.

What is claimed is:
 1. A method comprising: mounting a wafer-level package substrate over a carrier; pre-cutting the wafer-level package substrate to form trenches extending from a top surface of the wafer-level package substrate into the wafer-level package substrate; bonding a plurality of dies over the wafer-level package substrate; molding the plurality of dies in a molding material to form a wafer-level package, with the wafer-level package comprising the wafer-level package substrate, the plurality of dies, and the molding material; detaching the carrier from the wafer-level package; and sawing the wafer-level package into a plurality of packages, with each of the plurality of packages comprising a portion of the wafer-level package substrate and one of the plurality of dies.
 2. The method of claim 1, wherein the wafer-level package substrate comprises a core, a first dielectric layer overlying the core, and a second dielectric layer underlying the core, and wherein the trenches penetrate through the first dielectric layer and the core, and wherein bottom surfaces of the trenches are in the second dielectric layer.
 3. The method of claim 1, wherein the pre-cutting comprises a partial pre-cut, with bottom surfaces of the trenches being between a top surface and a bottom surface of the wafer-level package substrate.
 4. The method of claim 1, wherein the pre-cutting comprises a full pre-cut, with the trenches penetrating through the wafer-level package substrate.
 5. The method of claim 1, wherein portions of the molding material are filled into the trenches, and wherein after the sawing the wafer-level package, the portions of the molding material filled into the trenches have some portions left on the plurality of packages.
 6. The method of claim 1 further comprising bonding a plurality of additional dies, with each of the plurality of additional dies over and bonded to one of the plurality of dies, wherein one of the plurality of additional dies extends beyond an edge of a respective underlying one of the plurality of dies.
 7. The method of claim 1, wherein the carrier and the wafer-level package substrate have round top-view shapes.
 8. A method comprising: performing a partial pre-cutting on a wafer-level package substrate to form trenches in the wafer-level package substrate, wherein bottom surfaces of the trenches are between a top surface and a bottom surface of the wafer-level package substrate; after the pre-cutting, bonding a plurality of dies over the wafer-level package substrate; molding the plurality of dies in a molding material to form a wafer-level package, with the wafer-level package comprising the wafer-level package substrate, the plurality of dies, and the molding material; and sawing the wafer-level package into a plurality of packages.
 9. The method of claim 8 further comprising: before the partial pre-cutting, adhering a wafer-level package substrate to a carrier through an adhesive; and after the molding, detaching the carrier from the wafer-level package.
 10. The method of claim 9 further comprising, after the detaching the carrier and before the sawing, mounting solder balls on the wafer-level package substrate.
 11. The method of claim 8, wherein the wafer-level package substrate comprises a core, wherein the trenches penetrate through the core.
 12. The method of claim 8, wherein portions of the molding material are filled into the trenches, and wherein after the sawing the wafer-level package, the portions of the molding material filled into the trenches have some portions left on the plurality of packages.
 13. The method of claim 8 further comprising bonding a plurality of additional dies, with each of the plurality of additional dies over and bonded to one of the plurality of dies.
 14. The method of claim 13, wherein one of the plurality of additional dies extends beyond an edge of a respective underlying one of the plurality of dies.
 15. A package comprising: a package substrate comprising a top surface and a bottom surface; a first die over and bonded to the package substrate; and a molding material molding the first die therein, wherein the molding material comprises: a first portion encircling the first die; and a second portion underlying and connected to the first portion, wherein the second portion encircles a top portion of the package substrate, and wherein a bottom surface of the second portion of the molding material is at an intermediate level between the top surface and the bottom surface of the package substrate.
 16. The package of claim 15, wherein the package substrate comprises: a core, wherein the bottom surface of the second portion of the molding material is level with or lower than a bottom surface of the core; a first dielectric layer overlying the core; first redistribution lines in the first dielectric layer; a second dielectric layer underlying the core; and second redistribution lines in the second dielectric layer.
 17. The package of claim 16, wherein the core comprises: a dielectric layer; and a plurality of conductive pipes penetrating through the dielectric layer.
 18. The package of claim 15 further comprising a second die over and bonded to the first die, wherein the second die extends beyond an edge of the first die.
 19. The package of claim 15, wherein the package substrate has a thickness, and wherein the second portion of the molding material has a height greater than about a half of the thickness of the package substrate.
 20. The package of claim 15, wherein the package substrate comprises a portion overlapped by the second portion of the molding material. 